More accurately, no one understood how much exposure banks had to borrowers who over-borrowed, through their own negligence or being defrauded or blackmailed into it (e.g. Countrywide.)
Really, the three parties most directly responsible are Washington Mutual and Countrywide - who rightfully went out of business, and some of whose executives were rightly jailed - and HUD, for completely neglecting to enforce regulations concerning settlements. Of course, no one was held responsible for that.